ouch, that stings,
heres how i owuld fix it. you choose our own prefered method.
i would buy some foam insulation board. cut to close size and glue a couple pieces together to make it thick enough to carve with a rasp file into the shape you need to replace.so it sits under the existing fiberglass like a mold.
cover it smoothly in aluminum foil so the fiberglass resin doesn't melt it
tape and brace it into place after grinding and sanding the edges of the hole back to firm unbroken material.
then layer in fiberglass matting untill its built back up to where you need it.
grind and sand as close to shape as you can.
then with fiberglass filler smooth over any holes or gouges untill you have a good finish , sand and paint
dang tree's anyway<_<